Auto merge of #114457 - lcnr:trait_ref_is_knowable-normalize, r=compiler-errors

normalize in `trait_ref_is_knowable` in new solver

fixes rust-lang/trait-system-refactor-initiative#51

Alternatively we could avoid normalizing the self type and do this at the end of the `assemble_candidates_via_self_ty` stack by splitting candidates into:
- applicable without normalizing self type
- applicable for aliases, even if they can be normalized
- applicable for stuff which cannot get normalized further

I don't think this would have any significant benefits and it also seems non-trivial to avoid normalizing only the self type in `trait_ref_is_knowable`.

r? `@compiler-errors`

Author: bors

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -316,6 +316,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 
         self.assemble_param_env_candidates(goal, &mut candidates);
 
+        self.assemble_coherence_unknowable_candidates(goal, &mut candidates);
+
         candidates
     }
 
@@ -363,10 +365,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 
         self.assemble_object_bound_candidates(goal, &mut candidates);
 
-        self.assemble_coherence_unknowable_candidates(goal, &mut candidates);
-
         self.assemble_candidates_after_normalizing_self_ty(goal, &mut candidates, num_steps);
-
         candidates
     }
 
@@ -877,26 +876,43 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         goal: Goal<'tcx, G>,
         candidates: &mut Vec<Candidate<'tcx>>,
     ) {
+        let tcx = self.tcx();
         match self.solver_mode() {
             SolverMode::Normal => return,
-            SolverMode::Coherence => {
-                let trait_ref = goal.predicate.trait_ref(self.tcx());
-                match coherence::trait_ref_is_knowable(self.tcx(), trait_ref) {
-                    Ok(()) => {}
-                    Err(_) => match self
-                        .evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
-                    {
-                        Ok(result) => candidates.push(Candidate {
-                            source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
-                            result,
-                        }),
-                        // FIXME: This will be reachable at some point if we're in
-                        // `assemble_candidates_after_normalizing_self_ty` and we get a
-                        // universe error. We'll deal with it at this point.
-                        Err(NoSolution) => bug!("coherence candidate resulted in NoSolution"),
-                    },
+            SolverMode::Coherence => {}
+        };
+
+        let result = self.probe_candidate("coherence unknowable").enter(|ecx| {
+            let trait_ref = goal.predicate.trait_ref(tcx);
+
+            #[derive(Debug)]
+            enum FailureKind {
+                Overflow,
+                NoSolution(NoSolution),
+            }
+            let lazily_normalize_ty = |ty| match ecx.try_normalize_ty(goal.param_env, ty) {
+                Ok(Some(ty)) => Ok(ty),
+                Ok(None) => Err(FailureKind::Overflow),
+                Err(e) => Err(FailureKind::NoSolution(e)),
+            };
+
+            match coherence::trait_ref_is_knowable(tcx, trait_ref, lazily_normalize_ty) {
+                Err(FailureKind::Overflow) => {
+                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::OVERFLOW)
+                }
+                Err(FailureKind::NoSolution(NoSolution)) | Ok(Ok(())) => Err(NoSolution),
+                Ok(Err(_)) => {
+                    ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
                 }
             }
+        });
+
+        match result {
+            Ok(result) => candidates.push(Candidate {
+                source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
+                result,
+            }),
+            Err(NoSolution) => {}
         }
     }
 

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -388,44 +388,60 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             && is_normalizes_to_hack == IsNormalizesToHack::No
             && !self.search_graph.in_cycle()
         {
-            debug!("rerunning goal to check result is stable");
-            self.search_graph.reset_encountered_overflow(encountered_overflow);
-            let (_orig_values, canonical_goal) = self.canonicalize_goal(goal);
-            let Ok(new_canonical_response) = EvalCtxt::evaluate_canonical_goal(
-                self.tcx(),
-                self.search_graph,
-                canonical_goal,
-                // FIXME(-Ztrait-solver=next): we do not track what happens in `evaluate_canonical_goal`
-                &mut ProofTreeBuilder::new_noop(),
-            ) else {
-                bug!(
-                    "goal went from {certainty:?} to error: re-canonicalized goal={canonical_goal:#?} \
-                    first_response={canonical_response:#?},
-                    second response was error"
-                );
-            };
-            // We only check for modulo regions as we convert all regions in
-            // the input to new existentials, even if they're expected to be
-            // `'static` or a placeholder region.
-            if !new_canonical_response.value.var_values.is_identity_modulo_regions() {
-                bug!(
-                    "unstable result: re-canonicalized goal={canonical_goal:#?} \
-                    first_response={canonical_response:#?} \
-                    second_response={new_canonical_response:#?}"
-                );
-            }
-            if certainty != new_canonical_response.value.certainty {
-                bug!(
-                    "unstable certainty: {certainty:#?} re-canonicalized goal={canonical_goal:#?} \
-                     first_response={canonical_response:#?} \
-                     second_response={new_canonical_response:#?}"
-                );
-            }
+            // The nested evaluation has to happen with the original state
+            // of `encountered_overflow`.
+            let from_original_evaluation =
+                self.search_graph.reset_encountered_overflow(encountered_overflow);
+            self.check_evaluate_goal_stable_result(goal, canonical_goal, canonical_response);
+            // In case the evaluation was unstable, we manually make sure that this
+            // debug check does not influence the result of the parent goal.
+            self.search_graph.reset_encountered_overflow(from_original_evaluation);
         }
 
         Ok((has_changed, certainty, nested_goals))
     }
 
+    fn check_evaluate_goal_stable_result(
+        &mut self,
+        goal: Goal<'tcx, ty::Predicate<'tcx>>,
+        original_input: CanonicalInput<'tcx>,
+        original_result: CanonicalResponse<'tcx>,
+    ) {
+        let (_orig_values, canonical_goal) = self.canonicalize_goal(goal);
+        let result = EvalCtxt::evaluate_canonical_goal(
+            self.tcx(),
+            self.search_graph,
+            canonical_goal,
+            // FIXME(-Ztrait-solver=next): we do not track what happens in `evaluate_canonical_goal`
+            &mut ProofTreeBuilder::new_noop(),
+        );
+
+        macro_rules! fail {
+            ($msg:expr) => {{
+                let msg = $msg;
+                warn!(
+                    "unstable result: {msg}\n\
+                    original goal: {original_input:?},\n\
+                    original result: {original_result:?}\n\
+                    re-canonicalized goal: {canonical_goal:?}\n\
+                    second response: {result:?}"
+                );
+                return;
+            }};
+        }
+
+        let Ok(new_canonical_response) = result else { fail!("second response was error") };
+        // We only check for modulo regions as we convert all regions in
+        // the input to new existentials, even if they're expected to be
+        // `'static` or a placeholder region.
+        if !new_canonical_response.value.var_values.is_identity_modulo_regions() {
+            fail!("additional constraints from second response")
+        }
+        if original_result.value.certainty != new_canonical_response.value.certainty {
+            fail!("unstable certainty")
+        }
+    }
+
     fn compute_goal(&mut self, goal: Goal<'tcx, ty::Predicate<'tcx>>) -> QueryResult<'tcx> {
         let Goal { param_env, predicate } = goal;
         let kind = predicate.kind();

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -283,6 +283,37 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 
         Ok(self.make_ambiguous_response_no_constraints(maybe_cause))
     }
+
+    /// Normalize a type when it is structually matched on.
+    ///
+    /// For self types this is generally already handled through
+    /// `assemble_candidates_after_normalizing_self_ty`, so anything happening
+    /// in [`EvalCtxt::assemble_candidates_via_self_ty`] does not have to normalize
+    /// the self type. It is required when structurally matching on any other
+    /// arguments of a trait goal, e.g. when assembling builtin unsize candidates.
+    fn try_normalize_ty(
+        &mut self,
+        param_env: ty::ParamEnv<'tcx>,
+        mut ty: Ty<'tcx>,
+    ) -> Result<Option<Ty<'tcx>>, NoSolution> {
+        for _ in 0..self.local_overflow_limit() {
+            let ty::Alias(_, projection_ty) = *ty.kind() else {
+                return Ok(Some(ty));
+            };
+
+            let normalized_ty = self.next_ty_infer();
+            let normalizes_to_goal = Goal::new(
+                self.tcx(),
+                param_env,
+                ty::ProjectionPredicate { projection_ty, term: normalized_ty.into() },
+            );
+            self.add_goal(normalizes_to_goal);
+            self.try_evaluate_added_goals()?;
+            ty = self.resolve_vars_if_possible(normalized_ty);
+        }
+
+        Ok(None)
+    }
 }
 
 fn response_no_constraints_raw<'tcx>(

compiler/rustc_trait_selection/src/solve/search_graph/mod.rs

@@ -134,9 +134,13 @@ impl<'tcx> SearchGraph<'tcx> {
     /// Resets `encountered_overflow` of the current goal.
     ///
     /// This should only be used for the check in `evaluate_goal`.
-    pub(super) fn reset_encountered_overflow(&mut self, encountered_overflow: bool) {
-        if encountered_overflow {
-            self.stack.raw.last_mut().unwrap().encountered_overflow = true;
+    pub(super) fn reset_encountered_overflow(&mut self, encountered_overflow: bool) -> bool {
+        if let Some(last) = self.stack.raw.last_mut() {
+            let prev = last.encountered_overflow;
+            last.encountered_overflow = encountered_overflow;
+            prev
+        } else {
+            false
         }
     }
 

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -448,7 +448,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             // We need to normalize the b_ty since it's matched structurally
             // in the other functions below.
             let b_ty = match ecx
-                .normalize_non_self_ty(goal.predicate.trait_ref.args.type_at(1), goal.param_env)
+                .try_normalize_ty(goal.param_env, goal.predicate.trait_ref.args.type_at(1))
             {
                 Ok(Some(b_ty)) => b_ty,
                 Ok(None) => return vec![misc_candidate(ecx, Certainty::OVERFLOW)],
@@ -927,41 +927,4 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         let candidates = self.assemble_and_evaluate_candidates(goal);
         self.merge_candidates(candidates)
     }
-
-    /// Normalize a non-self type when it is structually matched on when solving
-    /// a built-in goal.
-    ///
-    /// This is handled already through `assemble_candidates_after_normalizing_self_ty`
-    /// for the self type, but for other goals, additional normalization of other
-    /// arguments may be needed to completely implement the semantics of the trait.
-    ///
-    /// This is required when structurally matching on any trait argument that is
-    /// not the self type.
-    fn normalize_non_self_ty(
-        &mut self,
-        mut ty: Ty<'tcx>,
-        param_env: ty::ParamEnv<'tcx>,
-    ) -> Result<Option<Ty<'tcx>>, NoSolution> {
-        if !matches!(ty.kind(), ty::Alias(..)) {
-            return Ok(Some(ty));
-        }
-
-        for _ in 0..self.local_overflow_limit() {
-            let ty::Alias(_, projection_ty) = *ty.kind() else {
-                return Ok(Some(ty));
-            };
-
-            let normalized_ty = self.next_ty_infer();
-            let normalizes_to_goal = Goal::new(
-                self.tcx(),
-                param_env,
-                ty::ProjectionPredicate { projection_ty, term: normalized_ty.into() },
-            );
-            self.add_goal(normalizes_to_goal);
-            self.try_evaluate_added_goals()?;
-            ty = self.resolve_vars_if_possible(normalized_ty);
-        }
-
-        Ok(None)
-    }
 }